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<a href="cfg__fwd_8hpp.html">Go to the documentation of this file.</a><div class="fragment"><div class="line"><a id="l00001" name="l00001"></a><span class="lineno">    1</span><span class="comment">/* Flow</span></div>
<div class="line"><a id="l00002" name="l00002"></a><span class="lineno">    2</span><span class="comment"> * Copyright 2023 Akamai Technologies, Inc.</span></div>
<div class="line"><a id="l00003" name="l00003"></a><span class="lineno">    3</span><span class="comment"> *</span></div>
<div class="line"><a id="l00004" name="l00004"></a><span class="lineno">    4</span><span class="comment"> * Licensed under the Apache License, Version 2.0 (the</span></div>
<div class="line"><a id="l00005" name="l00005"></a><span class="lineno">    5</span><span class="comment"> * &quot;License&quot;); you may not use this file except in</span></div>
<div class="line"><a id="l00006" name="l00006"></a><span class="lineno">    6</span><span class="comment"> * compliance with the License.  You may obtain a copy</span></div>
<div class="line"><a id="l00007" name="l00007"></a><span class="lineno">    7</span><span class="comment"> * of the License at</span></div>
<div class="line"><a id="l00008" name="l00008"></a><span class="lineno">    8</span><span class="comment"> *</span></div>
<div class="line"><a id="l00009" name="l00009"></a><span class="lineno">    9</span><span class="comment"> *   https://www.apache.org/licenses/LICENSE-2.0</span></div>
<div class="line"><a id="l00010" name="l00010"></a><span class="lineno">   10</span><span class="comment"> *</span></div>
<div class="line"><a id="l00011" name="l00011"></a><span class="lineno">   11</span><span class="comment"> * Unless required by applicable law or agreed to in</span></div>
<div class="line"><a id="l00012" name="l00012"></a><span class="lineno">   12</span><span class="comment"> * writing, software distributed under the License is</span></div>
<div class="line"><a id="l00013" name="l00013"></a><span class="lineno">   13</span><span class="comment"> * distributed on an &quot;AS IS&quot; BASIS, WITHOUT WARRANTIES OR</span></div>
<div class="line"><a id="l00014" name="l00014"></a><span class="lineno">   14</span><span class="comment"> * CONDITIONS OF ANY KIND, either express or implied.</span></div>
<div class="line"><a id="l00015" name="l00015"></a><span class="lineno">   15</span><span class="comment"> * See the License for the specific language governing</span></div>
<div class="line"><a id="l00016" name="l00016"></a><span class="lineno">   16</span><span class="comment"> * permissions and limitations under the License. */</span></div>
<div class="line"><a id="l00017" name="l00017"></a><span class="lineno">   17</span><span class="comment"></span> </div>
<div class="line"><a id="l00018" name="l00018"></a><span class="lineno">   18</span><span class="comment">/// @file</span></div>
<div class="line"><a id="l00019" name="l00019"></a><span class="lineno">   19</span><span class="comment"></span><span class="preprocessor">#pragma once</span></div>
<div class="line"><a id="l00020" name="l00020"></a><span class="lineno">   20</span> </div>
<div class="line"><a id="l00021" name="l00021"></a><span class="lineno">   21</span><span class="preprocessor">#include &quot;<a class="code" href="util__fwd_8hpp.html">flow/util/util_fwd.hpp</a>&quot;</span></div>
<div class="line"><a id="l00022" name="l00022"></a><span class="lineno">   22</span><span class="preprocessor">#include &lt;boost/program_options.hpp&gt;</span></div>
<div class="line"><a id="l00023" name="l00023"></a><span class="lineno">   23</span><span class="preprocessor">#include &lt;boost/filesystem.hpp&gt;</span></div>
<div class="line"><a id="l00024" name="l00024"></a><span class="lineno">   24</span><span class="preprocessor">#include &lt;boost/any.hpp&gt;</span></div>
<div class="line"><a id="l00025" name="l00025"></a><span class="lineno">   25</span><span class="preprocessor">#include &lt;vector&gt;</span></div>
<div class="line"><a id="l00026" name="l00026"></a><span class="lineno">   26</span><span class="preprocessor">#include &lt;string&gt;</span></div>
<div class="line"><a id="l00027" name="l00027"></a><span class="lineno">   27</span><span class="preprocessor">#include &lt;ostream&gt;</span></div>
<div class="line"><a id="l00028" name="l00028"></a><span class="lineno">   28</span><span class="comment"></span> </div>
<div class="line"><a id="l00029" name="l00029"></a><span class="lineno">   29</span><span class="comment">/**</span></div>
<div class="line"><a id="l00030" name="l00030"></a><span class="lineno">   30</span><span class="comment"> * Flow module that facilitates configuring modules, such as applications and APIs, via statically and/or dynamically</span></div>
<div class="line"><a id="l00031" name="l00031"></a><span class="lineno">   31</span><span class="comment"> * parsed sets of name/value pairs from config sources like files and command lines.  (It is also possible to use a</span></div>
<div class="line"><a id="l00032" name="l00032"></a><span class="lineno">   32</span><span class="comment"> * subset of the provided features to simplify option-related tasks even without parsing them from a file/etc.)</span></div>
<div class="line"><a id="l00033" name="l00033"></a><span class="lineno">   33</span><span class="comment"> *</span></div>
<div class="line"><a id="l00034" name="l00034"></a><span class="lineno">   34</span><span class="comment"> * ### Main concepts: your `Value_set`s and flow::cfg::Option_set ###</span></div>
<div class="line"><a id="l00035" name="l00035"></a><span class="lineno">   35</span><span class="comment"> * &quot;Configuration&quot; has lots of possible meanings, and flow::cfg certainly doesn&#39;t cover any huge range of what</span></div>
<div class="line"><a id="l00036" name="l00036"></a><span class="lineno">   36</span><span class="comment"> * could be called configuration.  Instead it is focused on a fairly common basic object type: a straightforward</span></div>
<div class="line"><a id="l00037" name="l00037"></a><span class="lineno">   37</span><span class="comment"> * `struct` of essentially scalar values.  (Composition/nested `struct`s are also supported, but don&#39;t worry about it</span></div>
<div class="line"><a id="l00038" name="l00038"></a><span class="lineno">   38</span><span class="comment"> * yet.)  Such `struct`s are often passed around larger programs and used to configure various modules.</span></div>
<div class="line"><a id="l00039" name="l00039"></a><span class="lineno">   39</span><span class="comment"> * It is typically a given that each type V for a data member in such a `struct`:</span></div>
<div class="line"><a id="l00040" name="l00040"></a><span class="lineno">   40</span><span class="comment"> *   - is copyable in a resonable way;</span></div>
<div class="line"><a id="l00041" name="l00041"></a><span class="lineno">   41</span><span class="comment"> *   - is comparable via `==` in a reasonable way.</span></div>
<div class="line"><a id="l00042" name="l00042"></a><span class="lineno">   42</span><span class="comment"> *</span></div>
<div class="line"><a id="l00043" name="l00043"></a><span class="lineno">   43</span><span class="comment"> * It is also reasonable that each member has some default value, set in the `struct` no-args ctor, so that</span></div>
<div class="line"><a id="l00044" name="l00044"></a><span class="lineno">   44</span><span class="comment"> * the full set of values in a default-cted instance of the `struct` represents a reasonable overall config for the</span></div>
<div class="line"><a id="l00045" name="l00045"></a><span class="lineno">   45</span><span class="comment"> * corresponding module.</span></div>
<div class="line"><a id="l00046" name="l00046"></a><span class="lineno">   46</span><span class="comment"> *</span></div>
<div class="line"><a id="l00047" name="l00047"></a><span class="lineno">   47</span><span class="comment"> * It is also often desirable to print dumps of the current contents of such a `struct`.  In that case we add</span></div>
<div class="line"><a id="l00048" name="l00048"></a><span class="lineno">   48</span><span class="comment"> * the requirement on each type V:</span></div>
<div class="line"><a id="l00049" name="l00049"></a><span class="lineno">   49</span><span class="comment"> *   - it has a standard `ostream` output `&lt;&lt;` operator.</span></div>
<div class="line"><a id="l00050" name="l00050"></a><span class="lineno">   50</span><span class="comment"> *</span></div>
<div class="line"><a id="l00051" name="l00051"></a><span class="lineno">   51</span><span class="comment"> * Often (though not always) one needs to *parse* into the member of this `struct`, from a stream/file</span></div>
<div class="line"><a id="l00052" name="l00052"></a><span class="lineno">   52</span><span class="comment"> * or perhaps command line (one or more *config sources*).  If this is indeed required then add the requirement on</span></div>
<div class="line"><a id="l00053" name="l00053"></a><span class="lineno">   53</span><span class="comment"> * each V:</span></div>
<div class="line"><a id="l00054" name="l00054"></a><span class="lineno">   54</span><span class="comment"> *   - it has a standard `istream` input `&gt;&gt;` operator.</span></div>
<div class="line"><a id="l00055" name="l00055"></a><span class="lineno">   55</span><span class="comment"> *</span></div>
<div class="line"><a id="l00056" name="l00056"></a><span class="lineno">   56</span><span class="comment"> * Sometimes such a `struct` is essentially *static*, meaning once the values are filled out (manually, or from</span></div>
<div class="line"><a id="l00057" name="l00057"></a><span class="lineno">   57</span><span class="comment"> * a config source), they shouldn&#39;t change.  More rarely they are *dynamic*, meaning</span></div>
<div class="line"><a id="l00058" name="l00058"></a><span class="lineno">   58</span><span class="comment"> * values can change from time to time, particularly on outside events like a SIGHUP triggering reading</span></div>
<div class="line"><a id="l00059" name="l00059"></a><span class="lineno">   59</span><span class="comment"> * values from a file.  In the latter case it is typical to wrap them in `shared_ptr`s; then a piece of code can</span></div>
<div class="line"><a id="l00060" name="l00060"></a><span class="lineno">   60</span><span class="comment"> * save a copy of such a handle to an immutable instance of the `struct`, wherein it can rely on a consistent</span></div>
<div class="line"><a id="l00061" name="l00061"></a><span class="lineno">   61</span><span class="comment"> * set of config, even while dynamic config updates will cause more such instances to be generated over time.</span></div>
<div class="line"><a id="l00062" name="l00062"></a><span class="lineno">   62</span><span class="comment"> * Hence add this requirement on the `struct` itself:</span></div>
<div class="line"><a id="l00063" name="l00063"></a><span class="lineno">   63</span><span class="comment"> *  - it should derive from util::Shared_ptr_alias_holder, so as to gain `Ptr` and `Const_ptr` aliases</span></div>
<div class="line"><a id="l00064" name="l00064"></a><span class="lineno">   64</span><span class="comment"> *    (to `shared_ptr&lt;&gt;`).</span></div>
<div class="line"><a id="l00065" name="l00065"></a><span class="lineno">   65</span><span class="comment"> *</span></div>
<div class="line"><a id="l00066" name="l00066"></a><span class="lineno">   66</span><span class="comment"> * Call the `struct` type satisfying the above requirements `Value_set`.  All facilities dealing with such value sets</span></div>
<div class="line"><a id="l00067" name="l00067"></a><span class="lineno">   67</span><span class="comment"> * are templates parameterized on `Value_set`.  Generally speaking it is straightforward to satisfy these</span></div>
<div class="line"><a id="l00068" name="l00068"></a><span class="lineno">   68</span><span class="comment"> * requirements; for example they hold for ~all built-in scalar types, plus `chrono::duration`, `std::string`,</span></div>
<div class="line"><a id="l00069" name="l00069"></a><span class="lineno">   69</span><span class="comment"> * and many more oft-used types; and deriving from `Shared_ptr_alias_holder` is just a line or two.</span></div>
<div class="line"><a id="l00070" name="l00070"></a><span class="lineno">   70</span><span class="comment"> *</span></div>
<div class="line"><a id="l00071" name="l00071"></a><span class="lineno">   71</span><span class="comment"> * Maintaining a `Value_set` like that is straighforward, but the things one wants to do with with an *entire*</span></div>
<div class="line"><a id="l00072" name="l00072"></a><span class="lineno">   72</span><span class="comment"> * `Value_set` -- as opposed to individual members in it -- tend to be laborious, anti-consistent, and error-prone</span></div>
<div class="line"><a id="l00073" name="l00073"></a><span class="lineno">   73</span><span class="comment"> * to do.  To wit:</span></div>
<div class="line"><a id="l00074" name="l00074"></a><span class="lineno">   74</span><span class="comment"> *   - stream output (tediously list members along with laborious and inconsistent `ostream&lt;&lt;` logic);</span></div>
<div class="line"><a id="l00075" name="l00075"></a><span class="lineno">   75</span><span class="comment"> *   - parsing (it&#39;s just hard -- and even with boost.program_options there&#39;s a ton of boiler-plate to write and</span></div>
<div class="line"><a id="l00076" name="l00076"></a><span class="lineno">   76</span><span class="comment"> *     tons of degrees of freedom);</span></div>
<div class="line"><a id="l00077" name="l00077"></a><span class="lineno">   77</span><span class="comment"> *     - documenting the available settings/meanings/semantics to the user (help message);</span></div>
<div class="line"><a id="l00078" name="l00078"></a><span class="lineno">   78</span><span class="comment"> *   - comparison `==` (tediously list `==` checks for each member);</span></div>
<div class="line"><a id="l00079" name="l00079"></a><span class="lineno">   79</span><span class="comment"> *   - validation/checking for valid values (tedious `if` statements and even more tedious stream output.</span></div>
<div class="line"><a id="l00080" name="l00080"></a><span class="lineno">   80</span><span class="comment"> *</span></div>
<div class="line"><a id="l00081" name="l00081"></a><span class="lineno">   81</span><span class="comment"> * Just about the only thing that *is* easy is simply accessing the values in a `Value_set`.  Indeed, this is the one</span></div>
<div class="line"><a id="l00082" name="l00082"></a><span class="lineno">   82</span><span class="comment"> * &quot;feature&quot; built-into `Value_set` that we want to leave available at all costs; it is concise, compile-time-checked,</span></div>
<div class="line"><a id="l00083" name="l00083"></a><span class="lineno">   83</span><span class="comment"> * and fast (all of which is less true or untrue of, say, a `map&lt;string, boost::any&gt;` and similar solutions).</span></div>
<div class="line"><a id="l00084" name="l00084"></a><span class="lineno">   84</span><span class="comment"> *</span></div>
<div class="line"><a id="l00085" name="l00085"></a><span class="lineno">   85</span><span class="comment"> * The main value of flow::cfg, as of this writing, is the Option_set class template.  `Option_set&lt;Value_set&gt;`</span></div>
<div class="line"><a id="l00086" name="l00086"></a><span class="lineno">   86</span><span class="comment"> * adds concise ways of doing all of the above in a streamlined way.  Even if you need not actually parse your</span></div>
<div class="line"><a id="l00087" name="l00087"></a><span class="lineno">   87</span><span class="comment"> * `Value_set` from a file, it still lets you concisely output, compare, validate the `Value_set`s themselves.</span></div>
<div class="line"><a id="l00088" name="l00088"></a><span class="lineno">   88</span><span class="comment"> *</span></div>
<div class="line"><a id="l00089" name="l00089"></a><span class="lineno">   89</span><span class="comment"> * Therefore see Option_set doc header.  Do realize that it is expected to be common-place to have multiple</span></div>
<div class="line"><a id="l00090" name="l00090"></a><span class="lineno">   90</span><span class="comment"> * `Value_set` types -- and therefore separate `Option_set&lt;&gt;` instances for each.  A server program, for example,</span></div>
<div class="line"><a id="l00091" name="l00091"></a><span class="lineno">   91</span><span class="comment"> * might start with just a `Static_value_set` for initial, immutable config; and later add a `Dynamic_value_set`</span></div>
<div class="line"><a id="l00092" name="l00092"></a><span class="lineno">   92</span><span class="comment"> * for settings that can change since startup.  Flow itself has flow::net_flow, wherein (e.g.)</span></div>
<div class="line"><a id="l00093" name="l00093"></a><span class="lineno">   93</span><span class="comment"> * net_flow::Peer_socket has a set of socket options, some static and some dynamically changeable once a connection</span></div>
<div class="line"><a id="l00094" name="l00094"></a><span class="lineno">   94</span><span class="comment"> * is open; hence it might have a `struct` for dynamic socket options and another for static.</span></div>
<div class="line"><a id="l00095" name="l00095"></a><span class="lineno">   95</span><span class="comment"> *</span></div>
<div class="line"><a id="l00096" name="l00096"></a><span class="lineno">   96</span><span class="comment"> * ### Config_manager ###</span></div>
<div class="line"><a id="l00097" name="l00097"></a><span class="lineno">   97</span><span class="comment"> * An Option_set is designed to be used in flexible combination with other `Option_set`s.  The variations on how</span></div>
<div class="line"><a id="l00098" name="l00098"></a><span class="lineno">   98</span><span class="comment"> * one might use them are equally as unpredictable and varied as how one might use `Value_set`-type `struct`s.</span></div>
<div class="line"><a id="l00099" name="l00099"></a><span class="lineno">   99</span><span class="comment"> *</span></div>
<div class="line"><a id="l00100" name="l00100"></a><span class="lineno">  100</span><span class="comment"> * Config_manager is an optional feature built on a couple of `Option_set`s, one for dynamic and one for static</span></div>
<div class="line"><a id="l00101" name="l00101"></a><span class="lineno">  101</span><span class="comment"> * config.  It exists to provide a simple but often-sufficient config manager for use by a server process.  Create</span></div>
<div class="line"><a id="l00102" name="l00102"></a><span class="lineno">  102</span><span class="comment"> * one of these, fill out your process&#39;s overall static and dynamic config `struct`s, and you should be able to</span></div>
<div class="line"><a id="l00103" name="l00103"></a><span class="lineno">  103</span><span class="comment"> * have a working config system with minimal boiler-plate, based on a static config file and command line; and</span></div>
<div class="line"><a id="l00104" name="l00104"></a><span class="lineno">  104</span><span class="comment"> * a separate dynamic config file (with the optional ability to set initial dynamic setting values from the</span></div>
<div class="line"><a id="l00105" name="l00105"></a><span class="lineno">  105</span><span class="comment"> * static config sources).</span></div>
<div class="line"><a id="l00106" name="l00106"></a><span class="lineno">  106</span><span class="comment"> *</span></div>
<div class="line"><a id="l00107" name="l00107"></a><span class="lineno">  107</span><span class="comment"> * If Config_manager is insufficient, one can build their own system on top of Option_set.</span></div>
<div class="line"><a id="l00108" name="l00108"></a><span class="lineno">  108</span><span class="comment"> *</span></div>
<div class="line"><a id="l00109" name="l00109"></a><span class="lineno">  109</span><span class="comment"> * @see Option_set class template, the main feature of flow::cfg.</span></div>
<div class="line"><a id="l00110" name="l00110"></a><span class="lineno">  110</span><span class="comment"> */</span></div>
<div class="line"><a id="l00111" name="l00111"></a><span class="lineno"><a class="line" href="namespaceflow_1_1cfg.html">  111</a></span><span class="keyword">namespace </span><a class="code hl_namespace" href="namespaceflow_1_1cfg.html">flow::cfg</a></div>
<div class="line"><a id="l00112" name="l00112"></a><span class="lineno">  112</span>{</div>
<div class="line"><a id="l00113" name="l00113"></a><span class="lineno">  113</span><span class="comment">// Types.</span></div>
<div class="line"><a id="l00114" name="l00114"></a><span class="lineno">  114</span> </div>
<div class="line"><a id="l00115" name="l00115"></a><span class="lineno">  115</span><span class="comment">// Find doc headers near the bodies of these compound types.</span></div>
<div class="line"><a id="l00116" name="l00116"></a><span class="lineno">  116</span> </div>
<div class="line"><a id="l00117" name="l00117"></a><span class="lineno">  117</span><span class="keyword">template</span>&lt;<span class="keyword">typename</span> Root, <span class="keyword">typename</span> Target,</div>
<div class="line"><a id="l00118" name="l00118"></a><span class="lineno">  118</span>         <span class="keyword">typename</span> Target_ptr</div>
<div class="line"><a id="l00119" name="l00119"></a><span class="lineno">  119</span>           = <span class="keyword">typename</span> std::pointer_traits&lt;typename Root::Ptr&gt;::template rebind&lt;Target const&gt;&gt;</div>
<div class="line"><a id="l00120" name="l00120"></a><span class="lineno">  120</span><span class="keyword">class </span><a class="code hl_class" href="classflow_1_1cfg_1_1Dynamic__cfg__context.html">Dynamic_cfg_context</a>;</div>
<div class="line"><a id="l00121" name="l00121"></a><span class="lineno">  121</span> </div>
<div class="line"><a id="l00122" name="l00122"></a><span class="lineno">  122</span><span class="keyword">class </span><a class="code hl_class" href="classflow_1_1cfg_1_1Option__set__base.html">Option_set_base</a>;</div>
<div class="line"><a id="l00123" name="l00123"></a><span class="lineno">  123</span> </div>
<div class="line"><a id="l00124" name="l00124"></a><span class="lineno">  124</span><span class="keyword">template</span>&lt;<span class="keyword">typename</span> Value_set&gt;</div>
<div class="line"><a id="l00125" name="l00125"></a><span class="lineno">  125</span><span class="keyword">class </span><a class="code hl_class" href="classflow_1_1cfg_1_1Option__set.html">Option_set</a>;</div>
<div class="line"><a id="l00126" name="l00126"></a><span class="lineno">  126</span> </div>
<div class="line"><a id="l00127" name="l00127"></a><span class="lineno">  127</span> </div>
<div class="line"><a id="l00128" name="l00128"></a><span class="lineno">  128</span><span class="comment">/* (The @namespace and @brief thingies shouldn&#39;t be needed, but some Doxygen bug necessitated them.</span></div>
<div class="line"><a id="l00129" name="l00129"></a><span class="lineno">  129</span><span class="comment"> * See flow::util::bind_ns for explanation... same thing here.) */</span></div>
<div class="line"><a id="l00130" name="l00130"></a><span class="lineno">  130</span><span class="comment"></span> </div>
<div class="line"><a id="l00131" name="l00131"></a><span class="lineno">  131</span><span class="comment">/**</span></div>
<div class="line"><a id="l00132" name="l00132"></a><span class="lineno"><a class="line" href="namespaceflow_1_1cfg_1_1fs.html">  132</a></span><span class="comment"> * @namespace flow::cfg::fs</span></div>
<div class="line"><a id="l00133" name="l00133"></a><span class="lineno">  133</span><span class="comment"> * @brief Short-hand for `namespace boost::filesystem`.</span></div>
<div class="line"><a id="l00134" name="l00134"></a><span class="lineno">  134</span><span class="comment"> */</span></div>
<div class="line"><a id="l00135" name="l00135"></a><span class="lineno">  135</span><span class="keyword">namespace </span><a class="code hl_namespace" href="namespaceboost_1_1filesystem.html">fs</a> = <a class="code hl_namespace" href="namespaceboost_1_1filesystem.html">boost::filesystem</a>;</div>
<div class="line"><a id="l00136" name="l00136"></a><span class="lineno">  136</span><span class="comment"></span> </div>
<div class="line"><a id="l00137" name="l00137"></a><span class="lineno">  137</span><span class="comment">/**</span></div>
<div class="line"><a id="l00138" name="l00138"></a><span class="lineno"><a class="line" href="namespaceflow_1_1cfg_1_1opts.html">  138</a></span><span class="comment"> * @namespace flow::cfg::opts</span></div>
<div class="line"><a id="l00139" name="l00139"></a><span class="lineno">  139</span><span class="comment"> * @brief Short-hand for `namespace boost::program_options`.</span></div>
<div class="line"><a id="l00140" name="l00140"></a><span class="lineno">  140</span><span class="comment"> */</span></div>
<div class="line"><a id="l00141" name="l00141"></a><span class="lineno">  141</span><span class="keyword">namespace </span>opts = boost::program_options;</div>
<div class="line"><a id="l00142" name="l00142"></a><span class="lineno">  142</span> </div>
<div class="line"><a id="l00143" name="l00143"></a><span class="lineno">  143</span><span class="comment">// Free functions.</span></div>
<div class="line"><a id="l00144" name="l00144"></a><span class="lineno">  144</span><span class="comment"></span> </div>
<div class="line"><a id="l00145" name="l00145"></a><span class="lineno">  145</span><span class="comment">/**</span></div>
<div class="line"><a id="l00146" name="l00146"></a><span class="lineno">  146</span><span class="comment"> * Serializes (briefly) an Option_set to a standard output stream.</span></div>
<div class="line"><a id="l00147" name="l00147"></a><span class="lineno">  147</span><span class="comment"> *</span></div>
<div class="line"><a id="l00148" name="l00148"></a><span class="lineno">  148</span><span class="comment"> * @relatesalso Option_set</span></div>
<div class="line"><a id="l00149" name="l00149"></a><span class="lineno">  149</span><span class="comment"> *</span></div>
<div class="line"><a id="l00150" name="l00150"></a><span class="lineno">  150</span><span class="comment"> * @tparam Value_set</span></div>
<div class="line"><a id="l00151" name="l00151"></a><span class="lineno">  151</span><span class="comment"> *         See Option_set doc header.</span></div>
<div class="line"><a id="l00152" name="l00152"></a><span class="lineno">  152</span><span class="comment"> * @param os</span></div>
<div class="line"><a id="l00153" name="l00153"></a><span class="lineno">  153</span><span class="comment"> *        Stream to which to serialize.</span></div>
<div class="line"><a id="l00154" name="l00154"></a><span class="lineno">  154</span><span class="comment"> * @param val</span></div>
<div class="line"><a id="l00155" name="l00155"></a><span class="lineno">  155</span><span class="comment"> *        Value to serialize.</span></div>
<div class="line"><a id="l00156" name="l00156"></a><span class="lineno">  156</span><span class="comment"> * @return `os`.</span></div>
<div class="line"><a id="l00157" name="l00157"></a><span class="lineno">  157</span><span class="comment"> */</span></div>
<div class="line"><a id="l00158" name="l00158"></a><span class="lineno">  158</span><span class="keyword">template</span>&lt;<span class="keyword">typename</span> Value_set&gt;</div>
<div class="line"><a id="l00159" name="l00159"></a><span class="lineno">  159</span>std::ostream&amp; <a class="code hl_function" href="namespaceflow_1_1cfg.html#af310e567f2763a9736eda686030e6311">operator&lt;&lt;</a>(std::ostream&amp; os, <span class="keyword">const</span> <a class="code hl_class" href="classflow_1_1cfg_1_1Option__set.html">Option_set&lt;Value_set&gt;</a>&amp; val);</div>
<div class="line"><a id="l00160" name="l00160"></a><span class="lineno">  160</span><span class="comment"></span> </div>
<div class="line"><a id="l00161" name="l00161"></a><span class="lineno">  161</span><span class="comment">/**</span></div>
<div class="line"><a id="l00162" name="l00162"></a><span class="lineno">  162</span><span class="comment"> * Utility, used by FLOW_CFG_OPTION_SET_DECLARE_OPTION() internally but made available as a public API</span></div>
<div class="line"><a id="l00163" name="l00163"></a><span class="lineno">  163</span><span class="comment"> * in case it is useful, that converts a string containing a conventionally formatted data member name into the</span></div>
<div class="line"><a id="l00164" name="l00164"></a><span class="lineno">  164</span><span class="comment"> * corresponding auto-determined config option name.</span></div>
<div class="line"><a id="l00165" name="l00165"></a><span class="lineno">  165</span><span class="comment"> *</span></div>
<div class="line"><a id="l00166" name="l00166"></a><span class="lineno">  166</span><span class="comment"> * @note An example for convenience, accurate as of the time of this writing:</span></div>
<div class="line"><a id="l00167" name="l00167"></a><span class="lineno">  167</span><span class="comment"> *       `m_cool_object.m_cool_sub_object-&gt;m_badass_sub_guy.m_cool_option_name` transforms to</span></div>
<div class="line"><a id="l00168" name="l00168"></a><span class="lineno">  168</span><span class="comment"> *       `cool-object.cool-sub-object.badass-sub-guy.cool-option-name`.</span></div>
<div class="line"><a id="l00169" name="l00169"></a><span class="lineno">  169</span><span class="comment"> *</span></div>
<div class="line"><a id="l00170" name="l00170"></a><span class="lineno">  170</span><span class="comment"> * The format for the contents of `member_id` shall be as follows: It shall consist of one or more</span></div>
<div class="line"><a id="l00171" name="l00171"></a><span class="lineno">  171</span><span class="comment"> * identifiers following the Flow coding guide, each starting with `m_`, concatenated with C++ object separator</span></div>
<div class="line"><a id="l00172" name="l00172"></a><span class="lineno">  172</span><span class="comment"> * sequences, each sequence chosen to be either `.` (object dereference) or `-&gt;` (pointer dereference).  `m_` for</span></div>
<div class="line"><a id="l00173" name="l00173"></a><span class="lineno">  173</span><span class="comment"> * each identifier is optional for this function -- though the coding guide requires it as of this writing anyway.</span></div>
<div class="line"><a id="l00174" name="l00174"></a><span class="lineno">  174</span><span class="comment"> *</span></div>
<div class="line"><a id="l00175" name="l00175"></a><span class="lineno">  175</span><span class="comment"> * Note that boost.program_options allows config files (when used as config sources, e.g,</span></div>
<div class="line"><a id="l00176" name="l00176"></a><span class="lineno">  176</span><span class="comment"> * Option_set::parse_config_file()) to declare a `[config-section]` which results in each `option-name` listed</span></div>
<div class="line"><a id="l00177" name="l00177"></a><span class="lineno">  177</span><span class="comment"> * in that section to be treated as-if named `config-section.option-name`.  This has synergy with nested objects</span></div>
<div class="line"><a id="l00178" name="l00178"></a><span class="lineno">  178</span><span class="comment"> * within a config value set being separated by dots also (or `-&gt;` if desired for orthogonal reasons, such as</span></div>
<div class="line"><a id="l00179" name="l00179"></a><span class="lineno">  179</span><span class="comment"> * if a smart pointer is used).</span></div>
<div class="line"><a id="l00180" name="l00180"></a><span class="lineno">  180</span><span class="comment"> *</span></div>
<div class="line"><a id="l00181" name="l00181"></a><span class="lineno">  181</span><span class="comment"> * @param member_id</span></div>
<div class="line"><a id="l00182" name="l00182"></a><span class="lineno">  182</span><span class="comment"> *        Identifier, perhaps obtained via the preprocessor `#feature` from an argument to a functional macro.</span></div>
<div class="line"><a id="l00183" name="l00183"></a><span class="lineno">  183</span><span class="comment"> * @return See above.</span></div>
<div class="line"><a id="l00184" name="l00184"></a><span class="lineno">  184</span><span class="comment"> */</span></div>
<div class="line"><a id="l00185" name="l00185"></a><span class="lineno">  185</span>std::string <a class="code hl_function" href="namespaceflow_1_1cfg.html#a696eee859b6e3f0fc0b6473a1d2836b0">value_set_member_id_to_opt_name</a>(<a class="code hl_typedef" href="namespaceflow_1_1util.html#ae4dc52186797094b3040379ff19124c9">util::String_view</a> member_id);</div>
<div class="line"><a id="l00186" name="l00186"></a><span class="lineno">  186</span><span class="comment"></span> </div>
<div class="line"><a id="l00187" name="l00187"></a><span class="lineno">  187</span><span class="comment">/**</span></div>
<div class="line"><a id="l00188" name="l00188"></a><span class="lineno">  188</span><span class="comment"> * Similar to value_set_member_id_to_opt_name() but used by FLOW_CFG_OPTION_SET_DECLARE_OPTION_KEYED() internally</span></div>
<div class="line"><a id="l00189" name="l00189"></a><span class="lineno">  189</span><span class="comment"> * (also made available as a public API in case it is useful), that does the job of</span></div>
<div class="line"><a id="l00190" name="l00190"></a><span class="lineno">  190</span><span class="comment"> * value_set_member_id_to_opt_name() in addition to substituting the last `[...]` fragment with</span></div>
<div class="line"><a id="l00191" name="l00191"></a><span class="lineno">  191</span><span class="comment"> * a dot separator, followed by the `ostream` encoding of `key`.</span></div>
<div class="line"><a id="l00192" name="l00192"></a><span class="lineno">  192</span><span class="comment"> *</span></div>
<div class="line"><a id="l00193" name="l00193"></a><span class="lineno">  193</span><span class="comment"> * @note An example for convenience, accurate as of the time of this writing:</span></div>
<div class="line"><a id="l00194" name="l00194"></a><span class="lineno">  194</span><span class="comment"> *       `m_cool_object.m_cool_sub_object-&gt;m_badass_sub_guy[cool_key].m_cool_option_name`, with `cool_key == 3`,</span></div>
<div class="line"><a id="l00195" name="l00195"></a><span class="lineno">  195</span><span class="comment"> *       transforms to `cool-object.cool-sub-object.badass-sub-guy.3.cool-option-name`.</span></div>
<div class="line"><a id="l00196" name="l00196"></a><span class="lineno">  196</span><span class="comment"> *</span></div>
<div class="line"><a id="l00197" name="l00197"></a><span class="lineno">  197</span><span class="comment"> * Behavior is undefined if the `[...]` part doesn&#39;t exist or is preceded or succeeded by nothing.</span></div>
<div class="line"><a id="l00198" name="l00198"></a><span class="lineno">  198</span><span class="comment"> * In reality for things to work as expected that part should also be followed by a C++ object separator</span></div>
<div class="line"><a id="l00199" name="l00199"></a><span class="lineno">  199</span><span class="comment"> * as in value_set_member_id_to_opt_name() doc header; so, e.g., `m_blah[idx]-&gt;m_blah` or `m_blah[idx].m_blah`.</span></div>
<div class="line"><a id="l00200" name="l00200"></a><span class="lineno">  200</span><span class="comment"> *</span></div>
<div class="line"><a id="l00201" name="l00201"></a><span class="lineno">  201</span><span class="comment"> * @tparam Key</span></div>
<div class="line"><a id="l00202" name="l00202"></a><span class="lineno">  202</span><span class="comment"> *         An `ostream&lt;&lt;`able type.  Common: `size_t` and `std::string`.</span></div>
<div class="line"><a id="l00203" name="l00203"></a><span class="lineno">  203</span><span class="comment"> * @param member_id</span></div>
<div class="line"><a id="l00204" name="l00204"></a><span class="lineno">  204</span><span class="comment"> *        Identifier, perhaps obtained via the preprocessor `#` feature from an argument to a functional macro.</span></div>
<div class="line"><a id="l00205" name="l00205"></a><span class="lineno">  205</span><span class="comment"> * @param key</span></div>
<div class="line"><a id="l00206" name="l00206"></a><span class="lineno">  206</span><span class="comment"> *        The value to splice in when replacing the key fragment inside `[]` (after the inserted `.`).</span></div>
<div class="line"><a id="l00207" name="l00207"></a><span class="lineno">  207</span><span class="comment"> * @return See above.</span></div>
<div class="line"><a id="l00208" name="l00208"></a><span class="lineno">  208</span><span class="comment"> */</span></div>
<div class="line"><a id="l00209" name="l00209"></a><span class="lineno">  209</span><span class="keyword">template</span>&lt;<span class="keyword">typename</span> Key&gt;</div>
<div class="line"><a id="l00210" name="l00210"></a><span class="lineno">  210</span>std::string <a class="code hl_function" href="namespaceflow_1_1cfg.html#aafaf432a612183beab72175831aaa960">value_set_member_id_to_opt_name_keyed</a>(<a class="code hl_typedef" href="namespaceflow_1_1util.html#ae4dc52186797094b3040379ff19124c9">util::String_view</a> member_id, <span class="keyword">const</span> Key&amp; key);</div>
<div class="line"><a id="l00211" name="l00211"></a><span class="lineno">  211</span><span class="comment"></span> </div>
<div class="line"><a id="l00212" name="l00212"></a><span class="lineno">  212</span><span class="comment">/**</span></div>
<div class="line"><a id="l00213" name="l00213"></a><span class="lineno">  213</span><span class="comment"> * Serializes a value of type `Value` to the given `ostream` suitably for output in Option_set-related output to user</span></div>
<div class="line"><a id="l00214" name="l00214"></a><span class="lineno">  214</span><span class="comment"> * such as in help messages.  The generic version simply forwards to `ostream&lt;&lt;` operator; but specializations/overloads</span></div>
<div class="line"><a id="l00215" name="l00215"></a><span class="lineno">  215</span><span class="comment"> * can massage/customize the output more suitably for usability.  The user may provide their own specializations or</span></div>
<div class="line"><a id="l00216" name="l00216"></a><span class="lineno">  216</span><span class="comment"> * overload on top of any already provided.</span></div>
<div class="line"><a id="l00217" name="l00217"></a><span class="lineno">  217</span><span class="comment"> *</span></div>
<div class="line"><a id="l00218" name="l00218"></a><span class="lineno">  218</span><span class="comment"> * @see E.g.: the `Value = chrono::duration` overload.</span></div>
<div class="line"><a id="l00219" name="l00219"></a><span class="lineno">  219</span><span class="comment"> * @tparam Value</span></div>
<div class="line"><a id="l00220" name="l00220"></a><span class="lineno">  220</span><span class="comment"> *         Type of `val`.  For this generic implementation `ostream &lt;&lt; Value` operator must exist.</span></div>
<div class="line"><a id="l00221" name="l00221"></a><span class="lineno">  221</span><span class="comment"> * @param os</span></div>
<div class="line"><a id="l00222" name="l00222"></a><span class="lineno">  222</span><span class="comment"> *        Stream to which to serialize.</span></div>
<div class="line"><a id="l00223" name="l00223"></a><span class="lineno">  223</span><span class="comment"> * @param val</span></div>
<div class="line"><a id="l00224" name="l00224"></a><span class="lineno">  224</span><span class="comment"> *        Value to serialize.</span></div>
<div class="line"><a id="l00225" name="l00225"></a><span class="lineno">  225</span><span class="comment"> */</span></div>
<div class="line"><a id="l00226" name="l00226"></a><span class="lineno">  226</span><span class="keyword">template</span>&lt;<span class="keyword">typename</span> Value&gt;</div>
<div class="line"><a id="l00227" name="l00227"></a><span class="lineno">  227</span><span class="keywordtype">void</span> <a class="code hl_function" href="namespaceflow_1_1cfg.html#a1f5ff766e52aae24e6d5c876b3b26e96">value_to_ostream</a>(std::ostream&amp; os, <span class="keyword">const</span> Value&amp; val);</div>
<div class="line"><a id="l00228" name="l00228"></a><span class="lineno">  228</span><span class="comment"></span> </div>
<div class="line"><a id="l00229" name="l00229"></a><span class="lineno">  229</span><span class="comment">/**</span></div>
<div class="line"><a id="l00230" name="l00230"></a><span class="lineno">  230</span><span class="comment"> * Overload that serializes a value of `chrono`-based `duration` including `Fine_duration` --</span></div>
<div class="line"><a id="l00231" name="l00231"></a><span class="lineno">  231</span><span class="comment"> * which is recommended to use for Option_set-configured time durations -- to the given `ostream` suitably for output</span></div>
<div class="line"><a id="l00232" name="l00232"></a><span class="lineno">  232</span><span class="comment"> * in Option_set-related output to user such as in help messages.  As of this writing it improves upon the default</span></div>
<div class="line"><a id="l00233" name="l00233"></a><span class="lineno">  233</span><span class="comment"> * `ostream&lt;&lt;` behavior by converting to coarser units without losing precision (e.g., not `&quot;9000000000 ns&quot;` but</span></div>
<div class="line"><a id="l00234" name="l00234"></a><span class="lineno">  234</span><span class="comment"> * `&quot;9 s&quot;`).</span></div>
<div class="line"><a id="l00235" name="l00235"></a><span class="lineno">  235</span><span class="comment"> *</span></div>
<div class="line"><a id="l00236" name="l00236"></a><span class="lineno">  236</span><span class="comment"> * @tparam Rep</span></div>
<div class="line"><a id="l00237" name="l00237"></a><span class="lineno">  237</span><span class="comment"> *         See `chrono::duration`.</span></div>
<div class="line"><a id="l00238" name="l00238"></a><span class="lineno">  238</span><span class="comment"> * @tparam Period</span></div>
<div class="line"><a id="l00239" name="l00239"></a><span class="lineno">  239</span><span class="comment"> *         See `chrono::duration`.</span></div>
<div class="line"><a id="l00240" name="l00240"></a><span class="lineno">  240</span><span class="comment"> * @param os</span></div>
<div class="line"><a id="l00241" name="l00241"></a><span class="lineno">  241</span><span class="comment"> *        Stream to which to serialize.</span></div>
<div class="line"><a id="l00242" name="l00242"></a><span class="lineno">  242</span><span class="comment"> * @param val</span></div>
<div class="line"><a id="l00243" name="l00243"></a><span class="lineno">  243</span><span class="comment"> *        Value to serialize.</span></div>
<div class="line"><a id="l00244" name="l00244"></a><span class="lineno">  244</span><span class="comment"> */</span></div>
<div class="line"><a id="l00245" name="l00245"></a><span class="lineno">  245</span><span class="keyword">template</span>&lt;<span class="keyword">typename</span> Rep, <span class="keyword">typename</span> Period&gt;</div>
<div class="line"><a id="l00246" name="l00246"></a><span class="lineno">  246</span><span class="keywordtype">void</span> <a class="code hl_function" href="namespaceflow_1_1cfg.html#a1f5ff766e52aae24e6d5c876b3b26e96">value_to_ostream</a>(std::ostream&amp; os, <span class="keyword">const</span> boost::chrono::duration&lt;Rep, Period&gt;&amp; val);</div>
<div class="line"><a id="l00247" name="l00247"></a><span class="lineno">  247</span><span class="comment"></span> </div>
<div class="line"><a id="l00248" name="l00248"></a><span class="lineno">  248</span><span class="comment">/**</span></div>
<div class="line"><a id="l00249" name="l00249"></a><span class="lineno">  249</span><span class="comment"> * Overload that serializes a list value (with `Element` type itself similarly serializable)</span></div>
<div class="line"><a id="l00250" name="l00250"></a><span class="lineno">  250</span><span class="comment"> * to the given `ostream` suitably for output in Option_set-related output to user such as in help messages.</span></div>
<div class="line"><a id="l00251" name="l00251"></a><span class="lineno">  251</span><span class="comment"> *</span></div>
<div class="line"><a id="l00252" name="l00252"></a><span class="lineno">  252</span><span class="comment"> * @tparam Element</span></div>
<div class="line"><a id="l00253" name="l00253"></a><span class="lineno">  253</span><span class="comment"> *         Any type for which `value_to_ostream(Element_type)` is available; but as of this writing</span></div>
<div class="line"><a id="l00254" name="l00254"></a><span class="lineno">  254</span><span class="comment"> *         it cannot itself be `std::vector&lt;&gt;` (no lists of lists).</span></div>
<div class="line"><a id="l00255" name="l00255"></a><span class="lineno">  255</span><span class="comment"> * @param os</span></div>
<div class="line"><a id="l00256" name="l00256"></a><span class="lineno">  256</span><span class="comment"> *        Stream to which to serialize.</span></div>
<div class="line"><a id="l00257" name="l00257"></a><span class="lineno">  257</span><span class="comment"> * @param val</span></div>
<div class="line"><a id="l00258" name="l00258"></a><span class="lineno">  258</span><span class="comment"> *        Value to serialize.</span></div>
<div class="line"><a id="l00259" name="l00259"></a><span class="lineno">  259</span><span class="comment"> */</span></div>
<div class="line"><a id="l00260" name="l00260"></a><span class="lineno">  260</span><span class="keyword">template</span>&lt;<span class="keyword">typename</span> Element&gt;</div>
<div class="line"><a id="l00261" name="l00261"></a><span class="lineno">  261</span><span class="keywordtype">void</span> <a class="code hl_function" href="namespaceflow_1_1cfg.html#a1f5ff766e52aae24e6d5c876b3b26e96">value_to_ostream</a>(std::ostream&amp; os, <span class="keyword">const</span> std::vector&lt;Element&gt;&amp; val);</div>
<div class="line"><a id="l00262" name="l00262"></a><span class="lineno">  262</span> </div>
<div class="line"><a id="l00263" name="l00263"></a><span class="lineno">  263</span>} <span class="comment">// namespace flow::cfg</span></div>
<div class="line"><a id="l00264" name="l00264"></a><span class="lineno">  264</span><span class="comment"></span> </div>
<div class="line"><a id="l00265" name="l00265"></a><span class="lineno">  265</span><span class="comment">/// We may add some ADL-based overloads into this namespace outside `flow`.</span></div>
<div class="line"><a id="l00266" name="l00266"></a><span class="lineno"><a class="line" href="namespaceboost_1_1filesystem.html">  266</a></span><span class="comment"></span><span class="keyword">namespace </span><a class="code hl_namespace" href="namespaceboost_1_1filesystem.html">boost::filesystem</a></div>
<div class="line"><a id="l00267" name="l00267"></a><span class="lineno">  267</span>{</div>
<div class="line"><a id="l00268" name="l00268"></a><span class="lineno">  268</span><span class="comment">// Free functions.</span></div>
<div class="line"><a id="l00269" name="l00269"></a><span class="lineno">  269</span><span class="comment"></span> </div>
<div class="line"><a id="l00270" name="l00270"></a><span class="lineno">  270</span><span class="comment">/**</span></div>
<div class="line"><a id="l00271" name="l00271"></a><span class="lineno">  271</span><span class="comment"> * ADL-based overload of boost.program_options `validate()` to allow for empty `boost::filesystem::path` values</span></div>
<div class="line"><a id="l00272" name="l00272"></a><span class="lineno">  272</span><span class="comment"> * in flow::cfg config parsing as well as any other boost.program_options parsing in the application.</span></div>
<div class="line"><a id="l00273" name="l00273"></a><span class="lineno">  273</span><span class="comment"> * Likely there&#39;s no need to call this directly: it is invoked by boost.program_options when parsing `path`s.</span></div>
<div class="line"><a id="l00274" name="l00274"></a><span class="lineno">  274</span><span class="comment"> *</span></div>
<div class="line"><a id="l00275" name="l00275"></a><span class="lineno">  275</span><span class="comment"> * More precisely: This has ~two effects:</span></div>
<div class="line"><a id="l00276" name="l00276"></a><span class="lineno">  276</span><span class="comment"> *   - flow::cfg::Option_set (and flow::cfg::Config_manager and anything else built on `Option_set`) will</span></div>
<div class="line"><a id="l00277" name="l00277"></a><span class="lineno">  277</span><span class="comment"> *     successfully parse an empty (or all-spaces) value for a setting of type `boost::filesystem::path`, resulting</span></div>
<div class="line"><a id="l00278" name="l00278"></a><span class="lineno">  278</span><span class="comment"> *     in a `path` equal to a default-constructed `path()`.</span></div>
<div class="line"><a id="l00279" name="l00279"></a><span class="lineno">  279</span><span class="comment"> *     - You may still disallow it via a validator expression in FLOW_CFG_OPTION_SET_DECLARE_OPTION() as usual.</span></div>
<div class="line"><a id="l00280" name="l00280"></a><span class="lineno">  280</span><span class="comment"> *       (Sometimes an empty &quot;path&quot; value is useful as a special or sentinel value.)  Such flow::cfg-user-specified</span></div>
<div class="line"><a id="l00281" name="l00281"></a><span class="lineno">  281</span><span class="comment"> *       individual-option-validator checking, internally, occurs after (and only if) the lower-level parsing already</span></div>
<div class="line"><a id="l00282" name="l00282"></a><span class="lineno">  282</span><span class="comment"> *       succeeded.</span></div>
<div class="line"><a id="l00283" name="l00283"></a><span class="lineno">  283</span><span class="comment"> *   - Any other use (even outside of flow::cfg) of boost.program_options to parse a `path` will now also allow</span></div>
<div class="line"><a id="l00284" name="l00284"></a><span class="lineno">  284</span><span class="comment"> *     an empty value (on command line, in file, etc.) in the same application.</span></div>
<div class="line"><a id="l00285" name="l00285"></a><span class="lineno">  285</span><span class="comment"> *</span></div>
<div class="line"><a id="l00286" name="l00286"></a><span class="lineno">  286</span><span class="comment"> * ### Rationale ###</span></div>
<div class="line"><a id="l00287" name="l00287"></a><span class="lineno">  287</span><span class="comment"> * Allowing empty `path`s in flow::cfg is required for usability.  It was a common use case to allow for a blank special</span></div>
<div class="line"><a id="l00288" name="l00288"></a><span class="lineno">  288</span><span class="comment"> * value for path settings.  However trying to in fact provide an empty value in a flow::cfg file (e.g., simply</span></div>
<div class="line"><a id="l00289" name="l00289"></a><span class="lineno">  289</span><span class="comment"> * `log-file=`) resulted in an &quot;Invalid argument&quot; error message and refusal of</span></div>
<div class="line"><a id="l00290" name="l00290"></a><span class="lineno">  290</span><span class="comment"> * flow::cfg::Option_set::parse_config_file() to successfully parse.</span></div>
<div class="line"><a id="l00291" name="l00291"></a><span class="lineno">  291</span><span class="comment"> *</span></div>
<div class="line"><a id="l00292" name="l00292"></a><span class="lineno">  292</span><span class="comment"> * (The following discussion is about implementation details and would normally be omitted from this public-facing API</span></div>
<div class="line"><a id="l00293" name="l00293"></a><span class="lineno">  293</span><span class="comment"> * doc header.  However, in this slightly unusual (for Flow) situation the solution happens to subtly affect</span></div>
<div class="line"><a id="l00294" name="l00294"></a><span class="lineno">  294</span><span class="comment"> * code outside of flow::cfg.  Therefore it is appropriate to discuss these internals here.)</span></div>
<div class="line"><a id="l00295" name="l00295"></a><span class="lineno">  295</span><span class="comment"> *</span></div>
<div class="line"><a id="l00296" name="l00296"></a><span class="lineno">  296</span><span class="comment"> * The reason for this problem: flow::cfg uses boost.program_options for config source (especially file) parsing.</span></div>
<div class="line"><a id="l00297" name="l00297"></a><span class="lineno">  297</span><span class="comment"> * By default, when parsing a string into the proper type `T` (here `T` being `path`),</span></div>
<div class="line"><a id="l00298" name="l00298"></a><span class="lineno">  298</span><span class="comment"> * boost.program_options uses `istream &gt;&gt; T` overload.  Turns out that reading a blank from a stream into `path` causes</span></div>
<div class="line"><a id="l00299" name="l00299"></a><span class="lineno">  299</span><span class="comment"> * the `istream` bad-bit to set, meaning `lexical_cast` throws `bad_lexical_cast`; boost.program_options then manifests</span></div>
<div class="line"><a id="l00300" name="l00300"></a><span class="lineno">  300</span><span class="comment"> * this as an arg parse error.  (If `T` were `std::string`, by contrast, no such problem would occur:</span></div>
<div class="line"><a id="l00301" name="l00301"></a><span class="lineno">  301</span><span class="comment"> * `istream &gt;&gt; string` will happily accept a blank string.)</span></div>
<div class="line"><a id="l00302" name="l00302"></a><span class="lineno">  302</span><span class="comment"> *</span></div>
<div class="line"><a id="l00303" name="l00303"></a><span class="lineno">  303</span><span class="comment"> * boost.program_options clearly suggests the proper way to custom-parse types at</span></div>
<div class="line"><a id="l00304" name="l00304"></a><span class="lineno">  304</span><span class="comment"> * https://www.boost.org/doc/libs/1_78_0/doc/html/program_options/howto.html#id-1.3.32.6.7 --</span></div>
<div class="line"><a id="l00305" name="l00305"></a><span class="lineno">  305</span><span class="comment"> * namely, define a `validate()` overload in the same namespace as type `T`, with a `T`-typed arg.</span></div>
<div class="line"><a id="l00306" name="l00306"></a><span class="lineno">  306</span><span class="comment"> * ADL (argument-dependent lookup) will then use that overload -- instead of the default one, which simply invokes</span></div>
<div class="line"><a id="l00307" name="l00307"></a><span class="lineno">  307</span><span class="comment"> * `lexical_cast` (i.e., `istream&gt;&gt;`) -- and reports an error if that throws.  This is the same technique used</span></div>
<div class="line"><a id="l00308" name="l00308"></a><span class="lineno">  308</span><span class="comment"> * to conveniently select `hash_value()` for `unordered_*`; `swap()` for STL containers; etc.</span></div>
<div class="line"><a id="l00309" name="l00309"></a><span class="lineno">  309</span><span class="comment"> *</span></div>
<div class="line"><a id="l00310" name="l00310"></a><span class="lineno">  310</span><span class="comment"> * This solves the problem.  However, somewhat unusually, this imposes the same more-permissive semantics on</span></div>
<div class="line"><a id="l00311" name="l00311"></a><span class="lineno">  311</span><span class="comment"> * *all* other uses of boost.program_options in any application linking Flow (which includes this overload).</span></div>
<div class="line"><a id="l00312" name="l00312"></a><span class="lineno">  312</span><span class="comment"> * This is arguably not-great; ideally we&#39;d affect flow::cfg and nothing else.  That said (1) there&#39;s no apparent</span></div>
<div class="line"><a id="l00313" name="l00313"></a><span class="lineno">  313</span><span class="comment"> * alternative in boost.program_options; and more to the point (2) this feels like either an improvement or</span></div>
<div class="line"><a id="l00314" name="l00314"></a><span class="lineno">  314</span><span class="comment"> * neutral.  If an empty path must be disallowed, this can be done via `notifier()` (or just a manual check).</span></div>
<div class="line"><a id="l00315" name="l00315"></a><span class="lineno">  315</span><span class="comment"> * So, all in all, this seemed fine.</span></div>
<div class="line"><a id="l00316" name="l00316"></a><span class="lineno">  316</span><span class="comment"> *</span></div>
<div class="line"><a id="l00317" name="l00317"></a><span class="lineno">  317</span><span class="comment"> * @param target</span></div>
<div class="line"><a id="l00318" name="l00318"></a><span class="lineno">  318</span><span class="comment"> *        `target` shall be loaded with the `path` on success.  Otherwise we shall throw, per required</span></div>
<div class="line"><a id="l00319" name="l00319"></a><span class="lineno">  319</span><span class="comment"> *        boost.program_options semantics.</span></div>
<div class="line"><a id="l00320" name="l00320"></a><span class="lineno">  320</span><span class="comment"> * @param user_values</span></div>
<div class="line"><a id="l00321" name="l00321"></a><span class="lineno">  321</span><span class="comment"> *        The (trimmed) raw strings from the user for this occurrence of the setting.</span></div>
<div class="line"><a id="l00322" name="l00322"></a><span class="lineno">  322</span><span class="comment"> *        In our case there must be only one, since a `path` has only one value.</span></div>
<div class="line"><a id="l00323" name="l00323"></a><span class="lineno">  323</span><span class="comment"> */</span></div>
<div class="line"><a id="l00324" name="l00324"></a><span class="lineno">  324</span><span class="keywordtype">void</span> <a class="code hl_function" href="namespaceboost_1_1filesystem.html#a5fa05fd030711d6d8c76139467b13256">validate</a>(boost::any&amp; target, <span class="keyword">const</span> std::vector&lt;std::string&gt;&amp; user_values, path*, <span class="keywordtype">int</span>);</div>
<div class="line"><a id="l00325" name="l00325"></a><span class="lineno">  325</span> </div>
<div class="line"><a id="l00326" name="l00326"></a><span class="lineno">  326</span>} <span class="comment">// namespace boost::filesystem</span></div>
<div class="ttc" id="aclassflow_1_1cfg_1_1Dynamic__cfg__context_html"><div class="ttname"><a href="classflow_1_1cfg_1_1Dynamic__cfg__context.html">flow::cfg::Dynamic_cfg_context</a></div><div class="ttdoc">Class which facilitates managing access to a dynamic configuration.</div><div class="ttdef"><b>Definition:</b> <a href="dynamic__cfg__context_8hpp_source.html#l00056">dynamic_cfg_context.hpp:57</a></div></div>
<div class="ttc" id="aclassflow_1_1cfg_1_1Option__set__base_html"><div class="ttname"><a href="classflow_1_1cfg_1_1Option__set__base.html">flow::cfg::Option_set_base</a></div><div class="ttdoc">Un-templated base for Option_set.</div><div class="ttdef"><b>Definition:</b> <a href="option__set_8hpp_source.html#l00033">option_set.hpp:34</a></div></div>
<div class="ttc" id="aclassflow_1_1cfg_1_1Option__set_html"><div class="ttname"><a href="classflow_1_1cfg_1_1Option__set.html">flow::cfg::Option_set</a></div><div class="ttdoc">The core config-parsing facility, which builds parsing/comparison/output capabilities on top of a giv...</div><div class="ttdef"><b>Definition:</b> <a href="option__set_8hpp_source.html#l00418">option_set.hpp:422</a></div></div>
<div class="ttc" id="anamespaceboost_1_1filesystem_html"><div class="ttname"><a href="namespaceboost_1_1filesystem.html">boost::filesystem</a></div><div class="ttdoc">We may add some ADL-based overloads into this namespace outside flow.</div><div class="ttdef"><b>Definition:</b> <a href="cfg__fwd_8hpp_source.html#l00266">cfg_fwd.hpp:267</a></div></div>
<div class="ttc" id="anamespaceboost_1_1filesystem_html_a5fa05fd030711d6d8c76139467b13256"><div class="ttname"><a href="namespaceboost_1_1filesystem.html#a5fa05fd030711d6d8c76139467b13256">boost::filesystem::validate</a></div><div class="ttdeci">void validate(boost::any &amp;target, const std::vector&lt; std::string &gt; &amp;user_values, path *, int)</div><div class="ttdoc">ADL-based overload of boost.program_options validate() to allow for empty boost::filesystem::path val...</div><div class="ttdef"><b>Definition:</b> <a href="option__set_8cpp_source.html#l00106">option_set.cpp:106</a></div></div>
<div class="ttc" id="anamespaceflow_1_1cfg_html"><div class="ttname"><a href="namespaceflow_1_1cfg.html">flow::cfg</a></div><div class="ttdoc">Flow module that facilitates configuring modules, such as applications and APIs, via statically and/o...</div><div class="ttdef"><b>Definition:</b> <a href="cfg__fwd_8hpp_source.html#l00111">cfg_fwd.hpp:112</a></div></div>
<div class="ttc" id="anamespaceflow_1_1cfg_html_a1f5ff766e52aae24e6d5c876b3b26e96"><div class="ttname"><a href="namespaceflow_1_1cfg.html#a1f5ff766e52aae24e6d5c876b3b26e96">flow::cfg::value_to_ostream</a></div><div class="ttdeci">void value_to_ostream(std::ostream &amp;os, const Value &amp;val)</div><div class="ttdoc">Serializes a value of type Value to the given ostream suitably for output in Option_set-related outpu...</div><div class="ttdef"><b>Definition:</b> <a href="option__set_8hpp_source.html#l02160">option_set.hpp:2160</a></div></div>
<div class="ttc" id="anamespaceflow_1_1cfg_html_a696eee859b6e3f0fc0b6473a1d2836b0"><div class="ttname"><a href="namespaceflow_1_1cfg.html#a696eee859b6e3f0fc0b6473a1d2836b0">flow::cfg::value_set_member_id_to_opt_name</a></div><div class="ttdeci">std::string value_set_member_id_to_opt_name(util::String_view member_id)</div><div class="ttdoc">Utility, used by FLOW_CFG_OPTION_SET_DECLARE_OPTION() internally but made available as a public API i...</div><div class="ttdef"><b>Definition:</b> <a href="option__set_8cpp_source.html#l00038">option_set.cpp:38</a></div></div>
<div class="ttc" id="anamespaceflow_1_1cfg_html_aafaf432a612183beab72175831aaa960"><div class="ttname"><a href="namespaceflow_1_1cfg.html#aafaf432a612183beab72175831aaa960">flow::cfg::value_set_member_id_to_opt_name_keyed</a></div><div class="ttdeci">std::string value_set_member_id_to_opt_name_keyed(util::String_view member_id, const Key &amp;key)</div><div class="ttdoc">Similar to value_set_member_id_to_opt_name() but used by FLOW_CFG_OPTION_SET_DECLARE_OPTION_KEYED() i...</div><div class="ttdef"><b>Definition:</b> <a href="option__set_8hpp_source.html#l02237">option_set.hpp:2237</a></div></div>
<div class="ttc" id="anamespaceflow_1_1cfg_html_af310e567f2763a9736eda686030e6311"><div class="ttname"><a href="namespaceflow_1_1cfg.html#af310e567f2763a9736eda686030e6311">flow::cfg::operator&lt;&lt;</a></div><div class="ttdeci">std::ostream &amp; operator&lt;&lt;(std::ostream &amp;os, const Option_set&lt; Value_set &gt; &amp;val)</div><div class="ttdoc">Serializes (briefly) an Option_set to a standard output stream.</div><div class="ttdef"><b>Definition:</b> <a href="option__set_8hpp_source.html#l02154">option_set.hpp:2154</a></div></div>
<div class="ttc" id="anamespaceflow_1_1util_html_ae4dc52186797094b3040379ff19124c9"><div class="ttname"><a href="namespaceflow_1_1util.html#ae4dc52186797094b3040379ff19124c9">flow::util::String_view</a></div><div class="ttdeci">Basic_string_view&lt; char &gt; String_view</div><div class="ttdoc">Commonly used char-based Basic_string_view. See its doc header.</div><div class="ttdef"><b>Definition:</b> <a href="string__view_8hpp_source.html#l00191">string_view.hpp:191</a></div></div>
<div class="ttc" id="autil__fwd_8hpp_html"><div class="ttname"><a href="util__fwd_8hpp.html">util_fwd.hpp</a></div></div>
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